The present invention relates to variable gain amplifiers. More particularly, the present invention relates to amplifiers with a high linearity and efficiency over a wide range of voltage levels that can be constructed in a variety of different topologies. One such topology is a grounded input active cascode configuration that increases gain while maintaining a high level of linearity. Another such topology is a composite NPN transistor configuration that realizes minimum complexity, high unity gain, and enhanced transconductance.
Amplifiers are used in a wide variety of applications. Generally, amplifiers are employed in applications where the voltage or current of a signal needs to be increased. One such application exists, for example, in communication transmitters where the voltage levels of audio signals are “weak” and need to be amplified so that they may be transmitted over relatively long distances. As per another example, a radio amplifies relatively small digital signals so that these signals may be heard by a user.
The simplest form of an amplifier is realized through the use of a single bipolar junction transistor (BJT). FIG. 1A illustrates a prior art single BJT amplifier 100 that is configured to increase input voltage 102 to output voltage 104. When amplifier 100 is in an active mode (e.g., amplifier 100 is ON), the current in the collector terminal of transistor 122 can be mapped to the current in the base terminal of transistor 122 with respect to an internal gain factor β. Specifically, the collector current of active mode transistor 122 is expressed as follows:ICOLLECTOR=(IBASE)*β
Similarly, the emitter current of transistor 122 when amplifier 100 is in an active mode can be mapped to the current in the collector of transistor 122 with respect to the internal gain factor β such that:IEMITTER=(ICOLLECTOR)*(β+1)/(β)
Amplifier 100 is constructed in a single-stage common-emitter configuration such that the emitter terminal is neither being utilized as the input nor the output of amplifier 100 (e.g., the emitter terminal is coupled to ground through resistor 112). Similarly, amplifier 100 can be constructed to be in a common-collector configuration (e.g., the base terminal receives an input signal and the emitter terminal provides an output signal) or a common-base configuration (e.g., the emitter terminal receives an input signal and the collector terminal provides an output signal).
As shown in FIG. 1A, amplifier 100 creates an amplified voltage differential across resistors 112 and 114 that is upwardly delimited by supply voltage 108. One disadvantage associated with using amplifier 100 relates to its inability to provide linear amplification over a substantial range of circuit conditions because it fails to preserve the original signal by a known gain, which is generally measured as the ratio of the output signal to the input signal of the amplifier, over the range of circuit conditions. Moreover, providing amplification with a known gain is important to avoid distortion of the signal being amplified. Single transistor prior art amplifiers do not exhibit a high level of linearity and, as a result, often introduce undesirable distortions into the amplified signal.
The transconductance (Gm) of an amplifier is a primary factor in modeling gain for that amplifier. The transconductance of amplifier 100 is the ratio of the change in current in the collector of transistor 122 to the change in voltage at the base terminal of transistor 122 over a defined, arbitrarily small, time interval. Linearity occurs when, for any two of the arbitrarily small intervals, the transconductance of an amplifier is constant. However, when a load is placed across differential 104, the transconductance of amplifier 100 changes for varying circuit conditions. As a result, amplifier 100 does not exhibit a high level of linearity across the full range of circuit conditions. Accordingly, it would be desirable to construct an amplifier that exhibits a high level of linearity across a full range of circuit conditions.
Furthermore, the voltage swing of amplifier 100 is severely delimited and has an output representative of a half-rectified wave. Only input voltages that are greater than the voltage threshold between the base terminal and emitter terminal of transistor 122 (e.g., voltages greater than the turn-ON voltage of transistor 122) will activate the transistor, allowing amplification to occur. The voltage headroom (e.g., the difference between the supply voltage and the maximum amplifier output voltage) of amplifier 100 is also limited by the point at which transistor 122 becomes saturated and by the impedance present at the collector terminal of transistor 122. Accordingly, it would be desirable to construct an amplifier with improved headroom.
As transistor 122 of amplifier 100 changes states, the charge transport that occurs inside transistor 122 is not instantaneous and the transistor's internal charge characteristics take time to rearrange. If an input signal changes faster than the internal charge profile of transistor 122 can rearrange itself, then the output of amplifier 100 will not be an amplified signal representative of the input signal. Thus, prior art amplifiers are greatly limited in the range of frequencies that they can successfully copy and enhance. For many applications, it would be desirable to construct an amplifier that can operate at high frequencies and in a focused bandwidth.
FIG. 1B shows another prior art single-BJT amplifier 150. In amplifier 150, the current applied to base terminal 154 of transistor 151 is amplified such that the current in collector terminal 152 is substantially equivalent to the current in base terminal 154 multiplied by the internal gain factor β of transistor 151. More particularly, the gain of amplifier 150 is as follows:I152=I154(β+1)
Resistor 156 and voltage source 158 may affect, or provide, the voltage located at base terminal 154 of transistor 151. Resistor 155 is utilized to control the transconductance of amplifier 150, which is defined as the current in collector terminal 152 divided by the voltage at base terminal 154. However, resistor 155 limits the range of signals that can be present at base terminal 154 of transistor 151, decreases gain (by increasing the voltage present at base terminal 154 of transistor 151), and contributes to noise degradation in amplifier 150. A compact amplifier circuit with increased frequency response, higher gain, and less internal noise is therefore desirable.
In order to stabilize linearity, feedback loops are occasionally integrated into an amplifier. Amplifier circuit 130 is illustrated in FIG. 1C and includes a negative current feedback loop which allows current at node 135 (e.g., the output of amplifier 131) to be drawn into node 134 (e.g., the inverting input of amplifier 131) by resistor 136. Amplifier 131 has a non-inverting input terminal connected to ground 139. Negative feedback loops may be utilized to help stabilize the linearity of an amplifier. As shown by example, the voltage across capacitor 138 and resistor 137 will be as follows:V138=V133(R136+R132)/R132However, in stabilizing linearity, amplifier circuit 130 does not provide a way to change its gain. As a result, compact amplifiers that include variable gain functionality are desired.
Amplifiers are frequently fabricated on an integrated semiconductor circuit (or chip). Such amplifiers are occasionally placed under different temperature conditions dependent upon the placement of an amplifier with respect to any heat producing components present on the chip. For example, an increased temperature exists around a chip's output nodes when these nodes exhibit HIGH voltages. The difference in temperature between two locations on a chip can be on the order of several degrees Celsius. Sub-micron amplifiers placed under different temperatures have different operating and performance characteristics. Therefore, it would be desirable to design amplifiers that exhibit the same operating and performance characteristics regardless of temperature.